Vendor supplied animals, animals received from collaborating investigators, animals raised at NIEHS, and animals under experiment in the Institute were examined for antibodies to several murine viruses. 1. F344 rats supplied by Harlan Industries were regularly positive for antibody to Sendai, PVM and RCV/SDA. Rats frequently had antibody to Sendai and PVM but not RCV/SDA on arrival; however, seroconversion to RCV/SDA and occasionally Sendai occurred in some rats held in isolation for 21 days. 2. Mice supplied by Harlan Industries and Research Triangle Institute had antibody to MHV. 3. Rats supplied by CRBL-Portage had antibody to KRV. 4. Mice received from Texas A and M were free of antibody to 11 murine viruses. 5. Colonies of mice and rats maintained at the CMB-RGR remained free of murine viruses (and other diseases). 6. Rats and mice in 16 rooms in Building 15 and 2 rooms in Building 2 were examined in June 1982 and February 1983 for antibody to murine viruses. Of 15 rooms examined in June 1982 for Sendai virus, 6 were positive (4/8 mice, 2/7 rat). Antibody to Sendai was not found in any animals in all 18 rooms in January 1983. Antibody to MHV was found in 5 of 8 rooms in both surveys. However, the same rooms were not positive on both surveys. Four of seven rooms were positive for RCV/SDA during the initial survey and 10 of 10 were positive during the subsequent survey. Mice (4/8 rooms) and rats (4/7 rooms) were initially positive for PVM. Subsequently 3/7 mouse rooms and 6/10 rat rooms were positive for PVM.